This invention relates to a process for preparing thermoinsulating and/or structural double-walled molded bodies and to the products thereby obtained.
More particularly, it relates to a process for preparing thermoinsulating and/or structural double-walled molded bodies by thermoforming two thermoplastic sheets on an expanded polymeric preform.
The process of the present invention does not employ blowing agents of chlorofluorocarbon nature.
With the term "molded body", as utilized in the present description and in the claims, it is intended whatever structural element, rigid, circular or polygonal, to be employed in the field of transports, household electric appliances, buildings, cars, telecommunications, office machines, etc.
Examples of suitable applications are doors, covers, frameworks particularly for refrigerators, freezer, panels, containers for portable thermic bags.
At present, the above mentioned molded articles are usually obtained starting from two half-shells which are mechanically assembled by soldering or sticking and afterwards the hollow part enclosed between the two half-shells is filled with expanded polyurethane.
According to another technique, an hollow case of a thermoplastic polymer is prepared by blow molding and is the filled with an expanded polyurethane mixture.
The expanded polyurethane employed as insulating material in both the above described techniques is obtained from a formulation constituted by an organic diisocyanate, a polyol, a siliconic surfactant, a polymerization catalyst and a blowing agent of chlorofluorocarbon nature.
At present, the use of expanded polyurethane as a thermoinsulating material presents ecological problems since it has been asserted that blowing agents of chlorofluorocarbon nature are one of the principal causes of the alteration and distruction of the ozone layer in the atmosphere.
However, the substitution of polyurethane with an equivalent material is a problem of difficult solution since by means of this polymer, expanded in situ with the well known R.I.M. (Reaction Injection Molding) technique, rigid self-carrying structural elements can be obtained, even though the outer case is made of thermoplastic polymer and not of metallic sheet.
This is due to the fact that polyurethane, reacting in situ, adheres perfectly to the inner walls of the case, thus forming with it a single structural body.